The present invention relates to a process for the activation of a catalyst. In particular, the present invention relates to a process for the activation of a hydrocarbon synthesis catalyst, with a hydrogen-containing gas, in the presence of a hydrocarbon liquid.
Hydrocarbon synthesis catalysts, that is catalysts capable of catalyzing synthesis of hydrocarbons from hydrogen and carbon monoxide (e.g. Fischer-Tropsch synthesis), typically comprise a Group VIII metal, supported on a catalyst carrier. The Group VIII metal is preferably chosen from iron, nickel, cobalt and/or ruthenium, more preferably iron or cobalt, especially cobalt. The catalyst carrier is preferably an inorganic refractory oxide, more preferably alumina, silica, titania, zirconia or mixtures thereof.
The Group VIII metal which is typically present in the hydrocarbon synthesis catalyst, in particular catalysts comprising iron, cobalt, nickel and/or ruthenium, should at least partly be in the metallic state in order to be active in catalyzing the synthesis of hydrocarbons from carbon monoxide and hydrogen. Thus, prior to use, the catalyst is subjected to one or more reduction, activation, step(s) in the presence of hydrogen.
Various ways to activate hydrocarbon synthesis catalysts are known in the art. Thus, European patent application publication No. 0 533 227 describes a process for the activation of a Fischer-Tropsch catalyst by contact with a hydrogen-containing gas, wherein the hydrogen concentration and the space velocity of the gas increase step-wise or continuously during the activation. European patent application publication No. 0 533 228 describes a process for the activation of a Fischer-Tropsch catalyst, which process comprises contacting the catalyst with a hydrogen-containing gas in a first stage at a total pressure up to 5 bar, rapidly increasing the pressure to at least 10 bar and contacting the catalyst with a hydrogen-containing gas in a second stage at this pressure. U.S. Pat. No. 4,670,414 describes an activation procedure comprising the steps, in sequence, of (a) reduction with a hydrogen-containing gas, (b) oxidation with an oxygen-containing gas, and (c) reduction with a hydrogen-containing gas.
The process for activating hydrocarbon synthesis catalysts may be performed ex-situ, but can also be performed in-situ in the reactor just prior to start-up, particularly for fixed bed units.
Hydrocarbon synthesis processes may be carried out in a number of catalyst bed types, such as fluidized beds, fixed beds, moving beds, ebullating beds and slurry beds. In ebullating and slurry beds in operation, the catalyst is kept dispersed in a liquid, typically a hydrocarbon liquid. Reactant gas bubbles (hydrogen and carbon monoxide) flow upwardly (usually) or downwardly through the catalyst-containing liquid.
It will be appreciated that it would be desirable to be able to activate the catalyst in the presence of the hydrocarbon liquid. This would in particular be desirable for slurry and ebullating catalyst beds. A major problem, however, is the occurrence of hydrogenolysis of the hydrocarbon liquid, which is catalyzed by the (partly) activated hydrocarbon synthesis catalyst. Hydrogenolysis of the hydrocarbon liquid may result in undesired methane formation and adiabatic temperature increase. Further, coke may form, affecting catalyst life and activity. The problem of hydrogenolysis especially applies to hydrocarbon synthesis catalysts comprising more than one metal. For instance, from U.S. Pat. No. 4,588,708 it appears that CO/Mn catalysts are about 15 times more active in the hydrogenolysis reaction than catalyst comprising cobalt only.
U.S. Pat. No. 5,292,705 discloses a process of activating a hydrocarbon synthesis catalyst, wherein the catalyst is first reduced (activated) ex-situ, without hydrocarbon liquids being present, and then the reduced catalyst is subsequently further activated in the presence of hydrogen and a hydrocarbon liquid. It is outlined in column 3, lines 15-23 of that publication that hydrogenolysis and coke formation is avoided in view of the relatively short treatment time.
European patent application publication No. 0 590 882 discloses a similar process in which a partly deactivated, but still reduced, catalyst is subjected to a rejuvenation treatment in the presence of a hydrocarbon liquid.
xe2x80x9cActivation,xe2x80x9d as used herein, is a process in which fresh catalyst is treated with hydrogen to reduce (oxidic) metal compounds to catalytically active metals, thereby activating the catalyst. Therefore, xe2x80x9cfresh,xe2x80x9d as used herein, refers to an unreduced or not yet reduced, oxygenate pre-catalyst which is not yet active. Usually, the catalyst is calcined by reaction with an oxygen containing gas at elevated temperatures before reduction.
xe2x80x9cRejuvenation,xe2x80x9d as used herein, is a process in which spent catalyst is treated with hydrogen to restore at least part of the initial activity of an activated fresh catalyst. Therefore, xe2x80x9cspent,xe2x80x9d as used herein, refers to a catalyst which is no longer active and which the metals have reverted to their oxidic, pre-catalyst state. Without wishing to be bound by a particular theory, it would appear that, inter alia, the processes which occur during rejuvenation are coke precursor removal, removal of metal-carrier compounds and reduction of metal compounds.
It would be desirable to be able to fully activate or rejuvenate catalyst, in particular a hydrocarbon synthesis catalyst, in the presence of a hydrocarbon liquid, whilst avoiding hydrogenolysis and/or coke formation, and obviating the need of a pre-reduction step.
It has now surprisingly been found possible to activate or rejuvenate catalysts in the presence of a hydrocarbon liquid, by contacting the catalysts with hydrogen or a hydrogen-containing gas in which the hydrogen partial pressure exceeds a certain limit.
The present invention relates to a process for the activation of a catalyst, preferably a hydrocarbon synthesis catalyst, in the presence of a hydrocarbon liquid which catalyst comprises a Group Ib, VIIb or VIII metal compound, by contacting a fresh or spent catalyst with a hydrogen-containing gas at a hydrogen partial pressure of at least 15 bar abs.